First of all, a frame structure of a wireless communication system is described with reference to FIG. 1 as follows.
FIG. 1 is a diagram for a frame structure of LTE (long term evolution) system.
Referring to FIG. 1, a single frame consists of 10 subframes. And, a single subframe consists of 2 slots. A time taken to transmit a single subframe is called a transmission time interval (hereinafter abbreviated TTI). For instance, a single subframe is 1 ms and a single slot is 0.5 ms.
A single slot consists of a plurality of OFDM (orthogonal frequency division multiplexing) symbols. In this case, the OFDM symbol can be called an SC-FDMA symbol or a symbols duration.
A single slot consists of 7 or 6 PFDM symbols according to a length of a cyclic prefix (hereinafter abbreviated CP). In LTE system, there are a normal CP and an extended CP. In case of using a normal CP, a single slot consists of 7 OFDM symbols. In case of using an extended CP, a single slot consists of 6 OFDM symbols.
Referring to FIG. 1, a primary synchronization channel (hereinafter abbreviated P-SCH) and a secondary synchronization channel (hereinafter abbreviated S-SCH) are transmitted for synchronization every frame. Moreover, in order to transmit resource allocation information of each subframe and the like every frame, a base station transmits a physical downlink control channel (hereinafter abbreviated PDCCH) on 0th to 2nd OFDM symbols. In doing so, it is able to send the PDCCH on 0th OFDM symbol, 0th and 1std OFDM symbols or 0th to 2nd OFDM symbols according to a quantity of the PDCCH. The number of OFDM symbols used by the PDCCH is variable each subframe. A base station informs a mobile station of the corresponding information via a physical control format indicator channel (hereinafter abbreviated PCFICH). Thus, the PCFICH should be transmitted each subframe and has total 3 kinds of informations. Table 1 shows a control format indicator (hereinafter abbreviated CFI). If CFI=1, it indicates that the PDCCH is transmitted on the 0th OFDM symbol only. If CFI=2, it indicates that the PDCCH is transmitted on the 0th and 1st OFDM symbols. If CFI=3, it indicates that the PDCCH is transmitted on the 0th to 2nd OFDM symbols.
TABLE 1CFI codewordCFI<b0, b1, . . . , b31>1<0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0,1, 1, 0, 1, 1, 0, 1, 1, 0, 1>2<1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1,0, 1, 1, 0, 1, 1, 0, 1, 1, 0>3<1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1,1, 0, 1, 1, 0, 1, 1, 0, 1, 1>4<0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,(Reserved)0, 0, 0, 0, 0, 0, 0, 0, 0, 0>
The CFI can be defined different according to a bandwidth. For instance, if a bandwidth of system is smaller than a specific threshold, ‘CFI=1’, ‘CFI=2’ and ‘CFI=3’ can indicate that the PDCCH is transmitted on ‘2 OFDM symbols’, ‘3 OFDM symbols’ and ‘4 OFDM symbols’, respectively.
FIG. 2 is a diagram for a transmission format of PCFICH.
Referring to FIG. 2, a resource element group (hereinafter abbreviated REG) consists of 4 subcarriers, is constructed with data subcarriers except a reference signal (hereinafter abbreviated RS), and is generally transmitted by applying a transmit diversity scheme. Moreover, in order to avoid inter-cell interference, a position of the REG is frequency shifted to use. Since PCFICH is always transmitted on a first OFDM symbol of a subframe, if a receiver receives a subframe, the receiver checks information of PCFICH first and then checks information of PDCCH. A size and usage of the PDCCH vary according to a format of a downlink control indicator (hereinafter abbreviated DCI). In particular, the size of the PDCCH can vary according to a coding rate. For instance, the DCI format can be defined as Table 2.
TABLE 2DCIformatObjectives0Scheduling of PUSCH1Scheduling of one PDSCH codeword1ACompact scheduling of one PDSCH codeword1BClosed-loop single-rank transmission1CPaging, RACH response and dynamic BCCH2Downlink scheduling of closed-loop rank-adapted spatialmultiplexing mode2ADownlink scheduling of open-loop rank-adapted spatialmultiplexing3TPC commands for PUCCH and PUSCH with 2 bitpower adjustments3ATPC commands for PUCCH and PUSCH with singlebit power adjustments
The DCI format shown in Table 2 is independently applied per mobile station. And, PDCCHs of several mobile stations are multiplexed within a single subframe. Channel coding is independently performed on the multiplexed PDCCH of the mobile stations and cyclic redundancy check (hereinafter abbreviated CRC) is applied thereto. In doing so, in order to enable the corresponding mobile station to receive its PDCCH, a unique ID of each mobile station is applied to the CRC.
Yet, since the mobile station is not aware of a location of its PDCCH, the mobile station performs blind detection on all PDCCHs of the corresponding DCI format each subframe until receiving the PDCCH having its mobile station ID. In addition, DCI format 0 and DCI format 1A are designed to be always received by the mobile station without increasing complexity in a manner of being configured to have the same size of information bits. In particular, the DCI format 1A is used in a fallback mode and an MIMO transmission scheme is determined according to a transmitting antenna configuration. For instance, in 3GPP Rel-8 system, a base station is able to have 1, 2 or 4 transmitting antennas. When a physical downlink shared channel (hereinafter abbreviated PDSCH) is transmitted using the DCI format 1A, if a base station has 1 transmitting antenna, the PDSCH is transmitted as single input signal output (hereinafter abbreviated SISO). If a base station has 2 or 4 transmitting antennas, the PDSCH is transmitted using the MIMO diversity scheme (TxD) shown in Table 3. In Table 3, a row of a matrix indicating a diversity scheme indicates a transmitting antenna, while a column is usable as a frequency or time.
TABLE 32Tx4TxTransmission scheme for PDSCH (TxD)      1          2        ⁢      (                                        S            1                                                S            2                                                            -                          S              2              *                                                            S            1            *                                )        1          2        ⁢      (                                        S            1                                                S            2                                    0                          0                                      0                          0                                      S            3                                                S            4                                                            -                          S              2              *                                                            S            1            *                                    0                          0                                      0                          0                                      -                          S              4              *                                                            S            3            *                                )  
Data transmitted by the MIMO transmission scheme shown in Table 3 is demodulated using a cell specific reference signal (hereinafter abbreviated CRS). FIG. 3 is a diagram of a structure of CRS according to the number of transmitting antenna(s).
The CRC structure shown in FIG. 3 can be represented as a single resource block (hereinafter abbreviated RB) within a single subframe shown in FIG. 1. In particular, N resource blocks construct a single subframe. And, a value of N is determined according to a system bandwidth.
Reference signal used for MIMO transmission scheme can be defined as an antenna port. In case of the above described SISO, it can be described that the antenna port 0 is used. Each antenna port can be defined as a cell only reference signal or a mobile station only reference signal. In case of the mobile station only reference signal, a corresponding layer is just decoded via the corresponding reference signal without description of special MIMO transmission scheme.
FIG. 4 is a diagram for a structure of antenna ports 0 to 3 (R0 to R3) corresponding to cell only reference signals in case of a normal CP. FIG. 5 is a diagram for a structure of antenna ports 0 to 3 (R0 to R3) corresponding to cell only reference signals in case of an extended normal CP.
And, FIG. 6 and FIG. 7 are diagrams of antenna ports 5, 7 and 8 (R5, R7, R8) corresponding to mobile station only reference signals.
In case of the mobile station reference signals shown in FIG. 6 and FIG. 7, they are characterized in that a mobile station is able to perform demodulation without description o special MIMO transmission scheme.
Meanwhile, a single radio frame can include M MBSFN subframes. In this case, a PDSCH region of the MBSFN subframe does not include a CRS. In case of Rel-9 mobile station, it is set not to receive PDSCH in the MBSFN subframe. Since the corresponding PDSCH region does not include the CRS, estimation for channel state information is not performed as well. Yet, in case of Rel-10 mobile station, a precoded demodulation reference signal (RS) is used and can be configured to transmit data on the PDSCH region of the MBSFN subframe. Although this subframe is recognized as the MBSFN subframe by the Rel-8 mobile station, it is recognized as a subframe capable of carrying PDSCH by the Rel-10 mobile station. This is named ‘LTE-A only subframe’ in the following description.